Recently, the hydroisomerization reaction of long-chain normal-paraffin has played an important part in an oil refining process. Long-chain normal-paraffin must be converted into iso-paraffin by a hydroisomerization reaction in order to improve cold flow properties at a low temperature and increase octane number. In particular, lately, the quality of a raw material has been deteriorated due to an increase in oil prices, whereas higher-quality fuel oil and lubricant products have been required due to the advancement of automobile engine technologies.
According to an example of the isomerization reaction used in an oil refining process, a C4 to C7 isomerized hydrocarbon can be applied to a process of manufacturing gasoline having a high octane number. The isomerization reaction of a C7 to C15 hydrocarbon can be practically used to manufacture high-grade diesel oil having a high cetane number and improved low-temperature cold flow properties. Further, the isomerization reaction of a normal C15 or more paraffin is put to practical use in the process of producing a high-grade lubricant having a high viscosity index. Particularly, since a commonly-used lubricant or jet oil needs low pour point and melting point, a technology for converting wax components using an isomerization reaction is required in order to produce a high-quality lubricant or jet oil. High boiling point, high molecular weight normal paraffins serving to increase the pour point coagulate to form a wax, and this wax must be removed for improving cold flow properties of the hydrocarbon feedstocks. Therefore, this hydroisomerization reaction for removing wax is referred to as “dewaxing”. In the dewaxing process, a hydroisomerization reaction is accompanied by a hydrocracking reaction. The term “hydroisomerization” is used when hydroisomerization predominates over hydrocracking, whereas the term “hydrocracking” is used when hydrocracking predominates over hydroisomerization. In this case, in order to maximize the yield of a product, it is advantageous for normal-paraffin to be converted by hydroisomerization, not by hydrocracking.
It is reported that an hydroisomerization reaction is generally conducted with a bifunctional catalyst. A bifunctional catalyst is composed of two kinds of active compositions of a metal site for hydrogenation and dehydrogenation and a acidic support for skeletal isomerization generating carbenium ions. Therefore, such a catalyst is referred to as a bifunctional catalyst. The metal composition used in the bifunctional catalyst may be at least one selected from group VI metals and group VIII metals. Particularly, precious metals having high hydrogenation and dehydrogenation activity, such as platinum (Pt), palladium (Pd) and the like, may be chiefly used as the metal composition. The support used in the bifunctional catalyst may be selected from various materials having an acidic site, such as silica-alumina, clay, zeolite and the like. Particularly, zeolite can be advantageously used as an isomerization catalyst because it is structurally stable even under a severe reaction condition, has a large surface area and includes a number of acid sites.
In order to produce a bifunctional catalyst for optimizing isomerization and controlling cracking, many researches and patents for improving the performance of a dewaxing catalyst using a zeolite material having excellent shape selectivity have been disclosed. Particularly, it was reported that, among various kinds of zeolite materials, each of the zeolite materials (ZSM-22, ZSM-23, EU-2, ZSM-48 and the like) having a one-dimensional pore structure are used as a support of a catalyst having high selectivity to isomerization. Further, it is disclosed in U.S. Pat. No. 7,482,300 that the selectivity of ZSM-48 to isomerization can be improved when the purity of the crystal structure of ZSM-48 becomes high.